How to fix mirror modifier doing weird things?
Is very likely that you have the object rotated in one or more axis. You can do 2 things (as far as I know):


Apply the rotation (keeping the current visual orientation): press, while in object mode, Ctrl+A and then select rotation.


Set the rotation values to 0 (this will rotate the object to a different orientation): in the object properties menu, in the transform tab set all the rotation values to 0.